Laboratory analysis to assess shale stability for the Zubair Formation, Southern Iraq

Abstract

The Zubair Formation consists of approximately 55% shale, which causes almost 70% of wellbore problems due to incompatibilities between drilling fluids and shale formations. The most common and effective solution to shale instability is through the design and selection of appropriate drilling fluids. Understanding the interaction between drilling fluids and shale has been a challenge due to the complexity of both the physical and chemical variations in shale formations. This paper presents some of the primary laboratory and wellsite testing techniques that are often used by mud engineers to characterize and remediate drilling fluids and shale interactions. Well-preserved core samples retrieved from the Zubair shale formation in Southern Iraq were run through extensive testing to describe the special characterization of the Zubair shale. These characteristics were measured and described, including the structure, texture, mineralogy, and reactivity, using a scanning electron microscope (SEM), a thin-section photograph, X-ray diffraction analysis (XRD) imaging, and cation exchange capacity (CEC) analysis. Moreover, a capillary suction timer (CST), hot rolling dispersion test, bulk hardness test, linear swell meter (LSM), and fracture development test were used to evaluate the stability of shale in the presence of test fluids. The test fluids included fresh water, 20 wt% NaCl brine, 7 wt% KCl brine, and a combination of 7 wt% KCl and 3 vol% glycol. The results illustrated that the Zubair shale is composed mainly (average content of 51.46%) of brittle minerals (i.e., quartz and calcite), along with 43.54% of clay minerals. The predominant clay minerals were kaolinite and illite, with an average content of 48.06% and 34.71%, respectively. In addition, the cation exchange capacity analysis and capillary suction time test indicated that Zubair shale has a low-to-moderate reactivity with drilling fluids. Furthermore, among the fluid systems tested, the best shale inhibition was achieved when the 7 wt% KCl and 3 vol% glycol solution was used. Shale sample analyses methods were used to understand the geologic features of the Zubair shale formations and to achieve a better perspective on the potential interactions of shale formations with drilling fluids. Understanding the properties and responses of shale formations to fluids is a significant step in achieving the chemical clay stabilization objectives. Proper design of drilling fluids, with appropriate mud weight and suitable additives, can lead to substantial cost reduction in drilling operations.